jit_expressions

Definition PG 18 manual

Determines whether expressions are JIT compiled, when JIT compilation is activated (see Section 30.2). The default is on.

How it works

jit_expressions controls whether expression evaluation is JIT compiled after the query has already crossed the top-level JIT cost gate. It does not activate JIT by itself.

Disabling it leaves expressions interpreted while other eligible JIT work can still occur. This isolates expression-code generation for testing correctness or compilation overhead.

The setting is a developer diagnostic rather than the normal way to tune JIT; jit_above_cost and the top-level jit switch govern production selection. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Leaving jit_expressions enabled after the bounded diagnostic or recovery task.

Running the experiment on the only copy of production data.

Underestimating log, core-file, temporary-file, WAL, CPU, or connection-slot amplification.

Treating a server that merely starts or completes a query as proof that data and behavior are correct.

Workload guidance

OLAP: Long analytical runs can amplify jit_expressions's debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.

OLTP: Do not tune production OLTP with jit_expressions. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.

SMALL: Keep jit_expressions at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.